1. Field of the Invention
The present invention relates generally to medical methods and devices. More particularly, the present invention relates to methods and apparatus for delivering and expanding prostheses composed of shape memory alloys within blood vessels and other body lumens.
In percutaneous transluminal coronary angioplasty (PTCA) procedures, a catheter having an expansible distal end, usually in the form of a balloon, is positioned in a lumen of a blood vessel with the distal end disposed within a stenotic atherosclerotic region of the vessel. The expansible end is then expanded to dilate the vessel and, upon withdrawal, restores adequate blood flow through the diseased region.
While angioplasty has gained wide acceptance, it continues to be limited by two major problems, abrupt closure and restenosis. Abrupt closure refers to the acute occlusion of a vessel immediately after or within the initial hours following the dilatation procedure. This complication occurs in approximately one of twenty cases and frequently results in myocardial infarction and death if blood flow is not quickly restored. Restenosis refers to the re-narrowing of an artery after an initially successful angioplasty. Occurring usually within the initial six months after angioplasty, and restenosis afflicts approximately one in three cases. That is, approximately one in three patients will require additional revascularization procedures.
Many different strategies have been tried with different degrees of success to reduce restenosis and abrupt closure, including pharmacologic (e.g., systemic and localized administration of anti-proliferative agents and other drugs) and mechanical (e.g., prolonged balloon inflations, atherectomy, laser angioplasty, post-angioplasty thermal conditioning, and stenting).
Of particular interest to the present invention, the intravascular delivery and implantation of stents to a blood vessel following balloon angioplasty procedures has proven to be of great value. The first stent to achieve widespread acceptance is the Palmaz-Schatz stent available from Johnson & Johnson Interventional Systems, a division of Ethicon, Inc., Somerville, N.J. The Palmaz-Schatz stent is a slotted tube formed from a malleable material. For delivery to the target site, the stent is usually placed over the balloon of a balloon delivery catheter having a non-distensible balloon. The angioplasty balloon catheter is then exchanged with the delivery catheter, and the stent positioned at the angioplasty treatment site. The balloon of the delivery catheter is then inflated to expand the stent in situ in order to implant the stent within the blood vessel.
A second class of stents is commonly referred to as "self-expanding stents" or "resilient stents" (in contrast to malleable stents as discussed above). This class of stent is defined primarily by the materials from which the stent is fabricated as well as the method in which the stent is retained and deployed. The materials of self-expanding stents may be resilient (spring-like) so that the stent may be delivered in a radially constrained state and implanted by releasing the stent from the constraint, whereby the stent springs back to its larger diameter configuration.
Of specific interest to the present invention, the stents may be formed from shape memory alloys where the stents are delivered in a reduced diameter configuration and subjected to conditions which cause a phase change in the material of the stent which in turn causes radial expansion of the stent structure within the blood vessel. Most commonly, such stents are formed from a nickel titanium alloy and are delivered in a deformed, smaller diameter configuration. Such stents are heated in situ to recover their original "memorized" larger diameter configuration. Typically, such alloys exhibit a crystallographic transformation from a martenistic structure at a lower temperature prior to delivery to an austenitic structure at a higher temperature to which they are subsequently exposed. In some cases, the stents are quickly heated to a temperature somewhat above body temperature when ready for deployment in the blood vessel lumen. In other cases, transformation to the austenitic phase will occur when the stent is exposed to body temperature.
At present, the use of shape memory stents which undergo a phase change and expand at a temperature between room temperature and body temperature is most common. Such stents may be stored at room temperature (although precautions should be taken to make sure that the phase change temperature is not exceeded) and will expand automatically when introduced to a blood vessel or other body lumen. In this way, there is no need to perform a separate deployment step, e.g. delivery of heated saline. Such methods are disadvantageous, however, in several respects. First, because of the relatively small temperature difference between room temperature and body temperature, it can be difficult to control the transition temperature of the stent material with sufficient accuracy. Moreover, the transition temperature being so close to room temperature requires that precautions be taken so that the transition temperature is not accidentally exceeded prior to deployment. Additionally, the stent will usually begin expanding while still contained within a delivery catheter or sheath, thus making final deployment and positioning of the stent problematic.
While the deficiencies just discussed are overcome by use of a stent which is deployed by heating to a temperature above body temperature, such stents and delivery systems suffer from a number of drawbacks of their own. For example, the delivery of a heated fluid through a catheter can be difficult to control, and it can be difficult to assure that the deployment temperature is reached. Use of saline above about 60.degree. C. and/or excessive volumes of heated saline can also cause vessel spasms and/or fibrillation of the heart. In fact, the extent of tissue damage typically increases almost exponentially as the temperature increases above 60 C. While it has been proposed to employ radio frequency energy to inductively heat shape memory stents to cause a phase change in transition, such energy can be deleterious to tissue and expose the patient to other forms of risk.
For these reasons, it would be desirable to provide improved methods and systems for delivering shape memory stents and prostheses to body lumens, such as blood vessels. In particular, it would be desirable to provide methods and systems for the in situ heating of stents, where the heating can be carefully controlled to open the shape memory stent in a desirable fashion. The methods and systems should provide for rapid and complete opening of the shape memory stents without subjecting the patient to undue risks associated with heat transfer. The heating methods should avoid collateral damage to the tissues and membranes surrounding the prosthesis to be expanded, and in particular should avoid inductive radio frequency exposure. The present invention is intended to address at least some of these concerns.
2. Description of the Background Art
Vascular stents and prosthesis composed from shape memory alloys, such as nitinol, are described in a number of patents and published applications including U.S. Pat. Nos. 3,868,956; 4,503,569; 4,795,458; 5,037,427; WO 94/22379; and EP 626 153. The use of heated saline for effecting a phase change and expanding a shape memory stent is disclosed in U.S. Pat. Nos. 4,795,458 and 5,037,427. The use of radio frequency induction for effecting a phase change and expanding a shape memory stent is disclosed in EP 626 153. Heated balloon catheters for performing thermally assisted angioplasty are described in U.S. Pat. Nos. 4,754,752 and 5,019,075.